The term "insole" as used herein is to be understood as referring to an inside sole of a shoe, particularly to a relatively thin insert introducible into a shoe and serving mainly for foot comfort.
The deleterious effect on the skeletal system, especially on the heel, the knee joint and the vertebral column, of repeated impact occasioned by running, jumping or even brisk walking, is well-known, with many attempts having been made to design special shoes or boots having built-in shock-absorbing members intended to alleviate this problem. Other attempts, such as described, for example, in U.S. Pat. No. 5,042,175, provide both integral, ie., built-in, and insole solutions. This U.S. patent proposes a sole (or an insole) containing a relatively large number of cylindrical compression springs, confined between a lower part of the sole or insole provided with a plurality of recesses nesting one end of these springs, and an upper cover strip removably attachable to the lower part.
The disadvantages of the above and similar soles or insoles resides largely in the fact that they all use cylindrical springs, the `solid` height of which, in other words, their height when compressed to the point where their turns rest on one another, is irreducible. As a consequence, in order to present a reasonable `working stroke,` the free, uncompressed length of such springs must be relatively large, causing such soles to be rather thick and cumbersome. With insoles that are intended for use with standard footwear, this problem is even more serious, as their thickness is liable to seriously reduce the free space of the shoe or boot.